Since the demonstration of the first layer-structured OLED with reasonable efficiency by Tang, et al, in “Organic Electroluminescent Diodes” in U.S. Pat. No. 4,769,292, OLED has become increasingly interesting technology for lighting applications, among other applications. The prior art on tandem OLEDs relates mostly to a field other than lighting applications, namely the field of displays, where a broadband white emission is required, the emission being separated into basic colour components by colour filters.
Typical OLEDs and electrically doped OLEDs have been described in detail, for example, in EP 1336208; tandem OLEDs, including electrically doped tandem OLEDs have been described in detail, for example, in EP 1804309. U.S. 2009/0191428 A1 discloses tandem OLED having fluorescent emitters, having both units comprising fluorescent emitters. U.S. 2008/0297036 A1 discloses a tandem white OLEDs, wherein both units comprise the same blue fluorescent emitter. U.S. 2008/0268282A1 discloses a tandem white OLED wherein one light emitting unit has 2 emission peaks and the other unit is white. U.S. 2007/0046189 A1 discloses an intermediate connector for stacked OLEDs. U.S. 2007/0001587 A1 and U.S. 2007/0001588 A1 U.S. 2006/0188745 A1 disclose tandem white OLEDs having two broadband light emitting units.
Notwithstanding all the developments made in the OLED field, there are continuing needs for OLED devices that provide higher power efficiencies or good colour for illumination purposes, or both.